<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <!--3.外部样式表-->
    <!--<link ret="pref="./outer.css">-->
    <!--2.内部样式表-->
    <style>
/*外部样式表，使用@import引入 必须放在style后面*/
        @import url(./outer.css);
        div{
            background-color: red;
        }
    </style>
</head>
<body>
    <!--内联样式-->
    <div style="width: 100px;height: 100px;">块级元素</div>
</body>
</html>